The investigation of cell lines derived from human teratocarcinomas can provide information pertinent to the characteristics of human embryonic cells, as well as knowledge essential for improving methods for clinically managing these tumors. Of crucial importance is the definition of the stem cells (presumed to be the embryonal carcinoma (EC) cells) and the analysis of their properties. Previously, we cloned a line of human EC cells, 2102Ep, which is able to undergo limited differentiation along a trophoblastic lineage in culture. These studies led to the demonstration that human EC cells, in contrast with their murine counterparts, express the cell surface antigen SSEA-3, but not SSEA-1, and also low levels of HLA-A,B,C and beta-2-microglobulin. Spontaneous differentiation under some culture conditions leads to the appearance of SSEA-1-positive cells, which synthesize fibronectin, and also a small number of cells with the ultrastructural features of trophoblastic giant cells and cytoplasmic HCG detectable by immunohistochemistry. Further work using recently derived monoclonal antibodies has now led to the identification of a new antigenic determinant, SSEA-4, which is also characteristically expressed by human EC cells. Both SSEA-3 and SSEA-4 epitopes are contained within a group of globoseries oligosaccharides occurring as glycosphinoglipids in the plasma membrane of these cells. They are thus related to the P-blood group system and are also found on red blood cells but few other cell types. Other monoclonal antibodies raised to human EC cells have been found to define two epitopes associated with the liver isozyme of alkaline phosphatase: an enzyme strongly expressed on the surface of human EC cells and a widely distributed antigen encoded by a gene on human chromosome 11. A second series of cloned human EC cells derived from the teratocarcinoma line TERA-2 have also been characterized in detail. These cells express the characteristic properties of human EC cells, but unlike the other available lines they are capable of differentiation into a variety of somatic cell types. Injected into nude mice they form tumors containing glandular structures and neural elements as well as nests of EC cells; in culture, little spontaneous differentiation occurs but they can be induced to differentiate into various cells, including neurons, by exposure to retinoic acid. This differentiation is irreversible and accompanied by the loss of the human EC cell characteristics and by the appearance of several other markers. Preliminary data suggest that at least two cell lineages arise following exposure to retinoic acid, and we are currently studying these by following the differential expression of a number of cell surface antigens. (M)